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[216.228.112.22]) by smtp.gmail.com with ESMTPSA id j5sm1616737pjs.2.2021.10.26.11.54.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 26 Oct 2021 11:54:18 -0700 (PDT) Date: Tue, 26 Oct 2021 11:54:16 -0700 From: Yury Norov To: Qian Cai Cc: Andy Shevchenko , Rasmus Villemoes , linux-kernel@vger.kernel.org Subject: Re: [PATCH] bitmap: simplify GENMASK(size - 1, 0) lines Message-ID: References: <20211026144108.35373-1-quic_qiancai@quicinc.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20211026144108.35373-1-quic_qiancai@quicinc.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 26, 2021 at 10:41:08AM -0400, Qian Cai wrote: > Since "size" is an "unsigned int", the rvalue "size - 1" will still be > "unsigned int" according to the C standard (3.2.1.5 Usual arithmetic > conversions). Therefore, GENMASK(size - 1, 0) will always return 0UL. Those > are also caught by GCC (W=2): > > ./include/linux/find.h: In function 'find_first_bit': > ./include/linux/bits.h:25:22: warning: comparison of unsigned expression in '< 0' is always false [-Wtype-limits] > 25 | __is_constexpr((l) > (h)), (l) > (h), 0))) > | ^ > ./include/linux/build_bug.h:16:62: note: in definition of macro 'BUILD_BUG_ON_ZERO' > 16 | #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); }))) > | ^ > ./include/linux/bits.h:25:3: note: in expansion of macro '__is_constexpr' > 25 | __is_constexpr((l) > (h)), (l) > (h), 0))) > | ^~~~~~~~~~~~~~ > ./include/linux/bits.h:38:3: note: in expansion of macro 'GENMASK_INPUT_CHECK' > 38 | (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l)) > | ^~~~~~~~~~~~~~~~~~~ > ./include/linux/find.h:119:31: note: in expansion of macro 'GENMASK' > 119 | unsigned long val = *addr & GENMASK(size - 1, 0); > | ^~~~~~~ > ./include/linux/bits.h:25:34: warning: comparison of unsigned expression in '< 0' is always false [-Wtype-limits] > 25 | __is_constexpr((l) > (h)), (l) > (h), 0))) > | ^ > ./include/linux/build_bug.h:16:62: note: in definition of macro 'BUILD_BUG_ON_ZERO' > 16 | #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); }))) > | ^ > ./include/linux/bits.h:38:3: note: in expansion of macro 'GENMASK_INPUT_CHECK' > 38 | (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l)) > | ^~~~~~~~~~~~~~~~~~~ > ./include/linux/find.h:119:31: note: in expansion of macro 'GENMASK' > 119 | unsigned long val = *addr & GENMASK(size - 1, 0); > | ^~~~~~~ > > Signed-off-by: Qian Cai > --- > include/linux/find.h | 28 ++++++++-------------------- > 1 file changed, 8 insertions(+), 20 deletions(-) > > diff --git a/include/linux/find.h b/include/linux/find.h > index 5bb6db213bcb..5ce2b17aea42 100644 > --- a/include/linux/find.h > +++ b/include/linux/find.h > @@ -115,11 +115,8 @@ unsigned long find_next_zero_bit(const unsigned long *addr, unsigned long size, > static inline > unsigned long find_first_bit(const unsigned long *addr, unsigned long size) > { > - if (small_const_nbits(size)) { > - unsigned long val = *addr & GENMASK(size - 1, 0); > - > - return val ? __ffs(val) : size; > - } > + if (small_const_nbits(size)) > + return size; > > return _find_first_bit(addr, size); > } [...] Nice catch! I'm a bit concerned that small_const_nbits() will never allow GENMASK() to be passed with size == 0, but the patch looks good to me overall. It's too late to merge it in 5.15, so I will add it in a spring merge window - most probably in a bitmap branch for 5.17 or 5.18: https://github.com/norov/linux/tree/bitmap-2022-Apr-01 Thanks, Yury